In U.S. Pat. No. 5,771,091, entitled "A Sensor and a Method for Measuring Distances to, and/or Physical Properties of, a Medium," there are disclosed various embodiments of such sensors in which the ring at the output end portion of the fiber connectable to a light source and the input end portion connectable to a light intensity measuring means are distant from each other, or at most, slightly touch each other. These sensors, however, suffer from some limitation of their sensitivity connected to the geometry of the disposition of the optical fibers as mounted in the optical heads, which affects the light intensity distribution in the optical fibers.
It is therefore an object of the present invention to overcome these limitations by changing the geometry of the optical fiber end portions.